DESCRIPTION: Viruses can use a number of strategies, including escape from immune recognition or induction of immunosuppression, to avoid immunological surveillance and thereby persist in the host. Elucidating the mechanisms by which viruses evade immune recognition is important to our understanding of viral pathogenesis and for the development of new therapeutic strategies for the treatment of viral infections such as hepatitis B and HIV. Viral persistence from infection of murine lymphocytic choriomeningitis virus (LCMV) can be achieved by specific tolerance ("clonal exhaustion") of antiviral cytotoxic T lymphocytes (CTLs) in a host with a mature immune system. We have shown that infection of mice with invasive strains of LCMV rapidly induces a large antigenic load and drives the activation and vigorous expansion of antigen specific CD8+ CTLs. After a transient phase of unresponsiveness (irreversible anergy), permanent deletion of CD8+ CTLs occurs. Since CD8+ CTLs are essential for virus elimination, their deletion in vivo results in the persistence of viral infection. Tolerance by "clonal exhaustion" does not affect virus specific CD4+ T cells since antibody production that is dependent on helper T cells in unaffected in these mice (split-tolerance). In this proposal, we wish to extend our previous findings by specifically focusing on the cellular and molecular mechanisms underlining "clonal exhaustion" of mature CTLs. The specific aims are the following: (1) To determine if destruction of the lymphoid tissue microenvironment by activated CTLs plays a role in the decay of the antiviral CTL response. Studies will be performed with mice deficient in perforin or in Fas/Fas-ligand dependent pathways which principally contribute to cytolysis of virally infected target cells. (2) To examine the fate of antiviral CTLs in lymphoid versus non-lymphoid tissues of mice with a persistent LCMV infection. The contribution of apoptosis (programmed cell death) and/or anergy (unresponsiveness to antigenic stimulation in vitro in the "clona exhaustion" of antiviral CTLs will be studied by quantitation and by functional analyses of responsive antiviral CTLs. This will be accomplished by using an adoptive transfer system with LCMV specific transgenic CD8+ CTLs as indicators. (3) To examine the role of CTL escape variants in the establishment of a persistent viral infection by "clonal exhaustion" of antiviral CTLs. The studies proposed will use mice infected with LCMV variants which lack CTL responses to one or to several dominant epitope peptides.